This invention relates to the field of membrane or panel keyboards. More particularly, this invention relates to new and improved membrane or panel keyboards having self-retention (i.e., key closure retention) structure and function.
A simple membrane or panel keyboard switch assembly includes a base sheet with electrically conductive contact elements thereon, an upper switch sheet with movable contact elements in a pattern corresponding to the base contact elements, and an insulating spacer sheet therebetween. An electrical connection (i.e. switch closure) is made by actuating a movable contact element and bringing it into physical contact with the corresponding base contact element. Depending upon the purpose and application of an instrument equipped with a panel keyboard, it is sometimes necessary to maintain a circuit closed (i.e. in a conducting state), either for a limited period or continuously, once contact has been made between the contact elements. In a simple panel keyboard switch, this can only be accomplished by continuously pushing the movable contact point pattern against the fixed contact point.
In the prior art, a separately provided special electric circuit has been used in order to achieve this condition, i.e., the self-retention or circuit closure condition. Such a circuit, however, is complex with many parts. As a result, the prior art self-retention or circuit closure or lock on devices incorporated into panel keyboards are both expensive and hinder the design freedom of such devices.